Just found this
In the perimenopausal years, decreased ovarian sensitivity to FSH and LH renders estrogen levels insufficient to produce the LH surge and ovulation. The corpus luteum cyst therefore does not form, so progesterone is not secreted. Progesterone not only stimulates ovulation, it blocks estrogen's ability to increase endometrial growth. In the absence of the LH surge, then, the follicles continue to produce estrogen, and the endometrium grows and thickens. Eventually the endometrium will outgrow its vascular support and shed, causing unpredictable medium-to-heavy bleeding.
Due to the lack of progesterone, there is no vasoconstriction of the spiral vessels within the endometrium and no orderly collapse to induce hemostasis as in normal menstrual bleeding. As a result, the bleeding can be spontaneous, heavy, and prolonged. Conversely, bleeding may be very light if estrogen release is too low to allow the endometrium to grow.